1. Field of The Invention
The present invention relates to methods and apparatus for inferential measurement; more particularly, to methods and apparatus for determining the magnetic permeability of a material; and most particularly, to a method and apparatus for using such measurement to control the concentration of a magnetic material in a magnetorheological (MR) fluid.
2. Discussion of the Related Art
MR fluids are well known and may be defined practically as fluid materials whose apparent viscosities are reversibly increased by exposure of the fluid to a magnetic field. The increase in viscosity is anisotropic, being greatest in the direction of the magnetic field due to formation of fibrils of magnetized particles. This property, known in the art as “stiffenening”, has been employed to great success in the field of extremely high resolution shaping, finishing, and polishing of surfaces, especially optical elements, wherein very small amounts of material may be removed in a highly precise and controlled manner. This field is known generally in the art as magnetorheological finishing (MRF). See, for example, U.S. Pat. Nos. 5,971,835; 6,746,310; and 6,893,322, the relevant disclosures of which are incorporated herein by reference.
A problem in the art of MRF is maintaining a constant magnetic particle concentration in the MR fluid entering the magnetic work zone. MR fluid is supplied to the work zone by a delivery system that draws MR fluid from a mixing sump into which used MR fluid passes from the work zone for mixing and reuse. The used MR fluid typically is depleted in carrier (water) by evaporation and also is heated, both of which alterations must be corrected before the MR fluid may be reused. Without replenishment of water lost to evaporation, the bulk supply of MR fluid in the sump will gradually increase in particle concentration during an MRF operation. This is an undesirable operating condition because particle concentration is an important factor governing the rate of removal of material from a substrate being finished. Thus, it is important to know what the particle concentration is in the MR fluid being supplied from the sump at any given time and to provide a proper water replenishment rate to the sump to replace the water lost to evaporation in use, thereby dynamically keeping the concentration constant at an aim value.
U.S. Pat. No. 5,554,932 discloses a system for measuring magnetic saturation flux density of a sample material. First and second sample holders are disposed symmetrically on either side of a cylindrical permanent magnet. Coils are placed around the sample holders and the permanent magnet is rotated. The signals induced in the coils in the absence of a magnetic material in one of the sample holders are applied to an amplifier/meter in such a manner as to provide a null signal. When a sample is placed in one of the sample holders, the magnetic saturation flux density can be measured. A shortcoming of the disclosed system is that the mechanical device is relatively cumbersome and has a critical moving part (the permanent magnet).
U.S. Pat. No. 6,650,108 discloses a system for inferring concentration of magnetic particles in a flowing MR fluid. The system is based on inductance measurement that converges in an impedance measurement with relatively complex technique involving high sensitivity electric bridge circuits. A shortcoming of the disclosed system is that resolution is relatively low.
U.S. patent application Ser. No. 11/681,258, filed Mar. 2, 2007, discloses a simple, high-resolution means for continuously measuring and monitoring the concentration of magnetic particles in the mixed sump MR fluid to permit controlled real-time dilution thereof before the sump MR fluid is reused for finishing. A shortcoming of the disclosed system is that the apparent concentration (magnetic permeability) is also a function of the electrical conductivity of the MR fluid.
What is needed in the art is a simple, high-resolution means for continuous compensation of output signal for changes in fluid conductivity in the mixed sump MR fluid to permit controlled real-time dilution thereof before the sump MR fluid is reused for finishing.
It is a principal object of the present invention to include consideration of fluid conductivity in determining, particle concentration in an MR fluid.